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Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Liabilities |  |
Redeemable convertible preferred stock warrants | $ 0 | $ 422
Money market funds |  |
Assets |  |
Investments | 15,223 | 22,115
U.S. treasury securities |  |
Assets |  |
Investments | 66,489 |
Corporate bonds |  |
Assets |  |
Investments | 19,720 |
Commercial paper |  |
Assets |  |
Investments | 18,529 |
Level 1 |  |
Liabilities |  |
Redeemable convertible preferred stock warrants | 0 | 0
Level 1 | Money market funds |  |
Assets |  |
Investments | 15,223 | 22,115
Level 1 | U.S. treasury securities |  |
Assets |  |
Investments | 66,489 |
Level 1 | Corporate bonds |  |
Assets |  |
Investments | 0 |
Level 1 | Commercial paper |  |
Assets |  |
Investments | 0 |
Level 2 |  |
Liabilities |  |
Redeemable convertible preferred stock warrants | 0 | 0
Level 2 | Money market funds |  |
Assets |  |
Investments | 0 | 0
Level 2 | U.S. treasury securities |  |
Assets |  |
Investments | 0 |
Level 2 | Corporate bonds |  |
Assets |  |
Investments | 19,720 |
Level 2 | Commercial paper |  |
Assets |  |
Investments | 18,529 |
Level 3 |  |
Liabilities |  |
Redeemable convertible preferred stock warrants | 0 | 422
Level 3 | Money market funds |  |
Assets |  |
Investments | 0 | $ 0
Level 3 | U.S. treasury securities |  |
Assets |  |
Investments | 0 |
Level 3 | Corporate bonds |  |
Assets |  |
Investments | 0 |
Level 3 | Commercial paper |  |
Assets |  |
Investments | $ 0 |

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